Biochemical correlates to circadian photoreceptor outer segment shedding were sought. Opsin synthesis as a function of the ambient light cycle was followed by studying radioactive leucine and glucosamine incorporation in vitro. Both precursors showed a diurinal rhythm of incorporation with a maximum occurring during the early morning hours prior to light onset. Glucosamine exhibited a more pronounced peak than leucine. This rhythm persisted when light-entrained animals were maintained in constant darkness indicating that opsin synthesis is a circadian rhythm that may be related to circadian photoreceptor shedding.